Contamination control, including particle monitoring, plays a critical role in the manufacturing processes of several industries. These industries require clean rooms or clean zones with active air filtration and require the supply of clean raw materials such as process gases, deionized water, chemicals, and substrates. In the pharmaceutical industry, the U.S. Food and Drug Administration requires particle monitoring because of the correlation between detected particles in an aseptic environment and viable and non-viable particles that contaminate the product being produced. Semiconductor fabrication companies require particle monitoring as an active part of quality control. As integrated circuits become more compact, line widths decrease, thereby reducing the size of particles that can cause quality problems. Accordingly, it is important to detect and accurately measure submicron-diameter particles of ever-decreasing sizes and numbers for each volumetric unit.
To perform particle monitoring, currently available commercial submicron-diameter particle detection systems use optical detection techniques to determine the presence, size, and number of particles in a volumetric unit. This technology is based on optical scattering of a light beam and detection of the optical signal after it has been scattered by a sample particle. The standard particle detection approach, which was developed during the late 1980s, entails intersecting, in a region referred to as a “view volume,” a light beam and a fluid stream containing sample particles. Light scattered by a particle in the view volume is collected with optics and focused onto a detection system that includes one or more detector elements. The detection system includes a light detector that detects the incidence of light and generates a pulse output signal, the magnitude of which depends on the intensity of the scattered light. The magnitude of the pulse output signal is compared to a predetermined pulse output signal threshold that is typically slightly above the average noise of the system. If the pulse output signal is less than the threshold, the signal is ignored. If the pulse output signal is greater than the threshold, the signal is processed by a computer that measures the voltage of the pulse output signal and determines particle size therefrom. Consequently, the ability of a particle detection system to detect small particles depends on its ability to distinguish between noise and pulse output signals generated from light scattered by submicron-diameter sample particles.
What is needed, therefore, is a particle detection system having high sensitivity in detecting submicron-diameter particles.